The SaaS company noticed something strange in their metrics. Signups were steady, but activation rates had dropped 40% over two weeks. Support tickets mentioned not receiving confirmation emails. The engineering team checked the logs—emails were sending successfully. No errors, no bounces, everything looked fine.
Except the emails weren't arriving. Gmail had quietly started routing them to spam. No notification, no warning, just a gradual slide into the spam folder that took weeks to notice and months to fix.
This is the insidious nature of deliverability problems. Your email infrastructure reports success because it successfully handed the email to the receiving server. What happens after that—inbox, spam, or the void—is invisible unless you're actively monitoring.
Deliverability monitoring tools provide that visibility. They show you where your emails actually land, track your sender reputation, and alert you when something goes wrong.
Inbox placement monitoring
GlockApps is built specifically for deliverability testing. Send your email to their seed list—real email addresses across major providers—and they'll report exactly where it landed. Gmail inbox? Outlook spam? Yahoo promotions tab? You'll know within minutes.
The seed list approach has limitations (you're testing specific addresses, not your actual recipients), but it's the closest thing to ground truth available. Their spam filter tests simulate how different providers evaluate your content, and the authentication checker verifies your SPF, DKIM, and DMARC setup.
Pricing starts around $59/month for basic monitoring, scaling up for higher volumes and more features. For organizations where email deliverability directly impacts revenue, the visibility is worth the cost.
Validity (formerly Return Path) offers enterprise-grade deliverability monitoring. Their Everest platform provides inbox placement data, reputation monitoring, and competitive benchmarking. The data comes from their massive panel of real email users who've opted in to share inbox data—giving insights into actual delivery patterns, not just seed list results.
The enterprise positioning means enterprise pricing. Validity targets large senders who need comprehensive deliverability intelligence. For smaller organizations, the cost may be prohibitive, but the data quality is unmatched.
250ok (now part of Validity) offered similar capabilities before the acquisition. If you're evaluating Validity, you're essentially getting the combined capabilities of both platforms.
Reputation monitoring
Sender Score from Validity provides a free reputation score for any IP address. Enter your sending IP, and you'll see a score from 0 to 100 based on complaint rates, spam trap hits, and other reputation signals. Scores above 80 generally indicate good reputation; below 70 suggests problems.
The free lookup is useful for spot checks, but the real value comes from ongoing monitoring. Reputation can change quickly—a bad campaign, a compromised account, or a sudden volume spike can tank your score. Regular monitoring catches problems before they cascade.
Google Postmaster Tools is essential for anyone sending to Gmail addresses (which is everyone). It shows how Gmail views your domain: spam rate, IP reputation, domain reputation, and authentication success rates. The data comes directly from Google, making it the authoritative source for Gmail deliverability.
Setup requires verifying domain ownership, but once configured, you get dashboards showing trends over time. A spike in spam rate or a drop in reputation is immediately visible. It's free, it's from Google, and there's no reason not to use it.
Microsoft SNDS (Smart Network Data Services) provides similar insights for Outlook and Hotmail. You'll see complaint rates, spam trap hits, and filter results for your sending IPs. The interface is less polished than Google's, but the data is equally valuable for Microsoft recipients.
Blacklist monitoring
MXToolbox monitors your sending IPs and domains against dozens of blacklists. Get alerted when you appear on a blacklist, with guidance on how to request removal. Their free tier checks against major blacklists; paid tiers add more comprehensive monitoring and faster alerts.
Blacklist appearance is often the first visible sign of a deliverability problem. By the time you notice delivery failures, you might have been blacklisted for days. Proactive monitoring catches listings early, when removal is easier.
Hetrix Tools offers blacklist monitoring as part of a broader uptime monitoring service. If you're already monitoring server uptime, adding blacklist checks to the same dashboard is convenient. Their free tier includes basic blacklist monitoring.
Authentication monitoring
Dmarcian focuses specifically on DMARC monitoring and reporting. DMARC generates XML reports that are nearly impossible to read manually. Dmarcian aggregates these reports into readable dashboards showing authentication pass/fail rates, sending sources, and potential spoofing attempts.
Beyond monitoring, they provide guidance on tightening your DMARC policy over time—moving from p=none (monitoring only) to p=quarantine to p=reject (full enforcement). The gradual approach prevents blocking legitimate email while you identify all authorized senders.
Valimail offers similar DMARC monitoring with a focus on automation. Their platform can automatically maintain SPF records as you add and remove email services, solving the common problem of SPF records becoming outdated. The automation is particularly valuable for organizations with many email-sending services.
Postmark's DMARC monitoring is free and straightforward. Send your DMARC reports to their service, and they'll provide weekly digests summarizing authentication results. It's less comprehensive than dedicated DMARC platforms, but the price (free) is right for basic monitoring.
Integrated monitoring
Many email service providers include deliverability monitoring in their platforms. Mailgun, SendGrid, and Postmark all provide dashboards showing delivery rates, bounce rates, and reputation metrics. If you're using one of these services, start with their built-in monitoring before adding external tools.
The limitation is that ESP monitoring only shows what happens within their system. They can tell you the email was accepted by Gmail's servers, but not whether it landed in inbox or spam. For that visibility, you need inbox placement tools like GlockApps or Validity.
Building a monitoring stack
No single tool provides complete visibility. A practical monitoring stack combines several:
Use Google Postmaster Tools and Microsoft SNDS for provider-specific reputation data. They're free and authoritative. Add MXToolbox or similar for blacklist monitoring. Use your ESP's built-in analytics for delivery and bounce rates.
For inbox placement visibility, GlockApps or similar seed list testing shows where emails actually land. Run tests before major campaigns and periodically for ongoing monitoring.
For DMARC, use Dmarcian, Valimail, or Postmark's free service to make sense of authentication reports. DMARC data reveals unauthorized senders and authentication failures that affect deliverability.
The goal is catching problems early. A small reputation dip is easier to fix than a major deliverability crisis. Monitoring provides the early warning that makes proactive fixes possible.
Frequently asked questions
How quickly can deliverability problems develop?
Very quickly. A single bad campaign—high complaint rates, spam trap hits, or sudden volume spikes—can damage reputation within hours. Recovery typically takes weeks to months. This is why monitoring matters: catching problems early limits damage.
What's a good spam complaint rate?
Below 0.1% (1 complaint per 1,000 emails) is the target. Above 0.3% triggers warnings from most providers. Above 0.5% causes serious deliverability problems. Google Postmaster Tools shows your Gmail complaint rate directly.
Do I need paid monitoring tools?
For basic monitoring, free tools (Google Postmaster, Microsoft SNDS, MXToolbox free tier) provide essential visibility. Paid tools add inbox placement testing, more comprehensive blacklist monitoring, and better alerting. The value depends on how critical email is to your business.
How often should I check deliverability metrics?
At minimum, weekly reviews of key metrics. Set up alerts for significant changes—reputation drops, blacklist appearances, spam rate spikes. Before major campaigns, run inbox placement tests. The goal is catching problems before they become crises.